m/\.swf(?:\?.*)?$/oi

\.        # the character '.'
swf       # the string, 'swf'
(?: ... ) # don't save this expression
\?        # the character '?'
.*        # zero or more characters (not a newline)
?         # zero or one occurances (of the grouping in parens)
$         # end of the line
/oi       # don't re-evaluate, case insensitive.
[download]

So, as best I can tell, you're not only grabbing URLs that end in SWF, you're also grabbing URLs that are passing a query string (ie, URL?QUERY_STRING format)